Resistance-variable memories (ReRAM: resistive random access memory) are nonvolatile memories in which the storage device part consists of a two-terminal structure in which a variable resistance layer is sandwiched by two electrodes. Since the cell structure is simple, scaling is easier than with other memories. For this reason, NAND type flash memories, which are utilized as large-capacity semiconductor storage devices, are promising candidates to be the next-generation large-capacity storage devices that will replace existing products.
As variable-resistance materials of the resistance-variable memories, various substances such as the transition metal oxide group, sulfide group, perovskite oxide group, and semiconductors have been used. To realize a large-capacity storage device using these resistance-variable memories, it is necessary to use a cross point type of memory structure.
In the cross point type of memory structure, a current leak (stray current) can result from a sneak current during write, readout, and erase operations. The occurrence of this leak current increases the power consumed by a device or hinders normal operations.
In order to avoid this problem, it is necessary to connect rectifying elements to resistance-variable elements in series, in order to provide a rectifying function to memory cells. However, since combining the rectifying elements such as diodes with the resistance-variable elements leads directly to an increase in the device size, it is difficult to attain a large capacity. Accordingly, it is more desirable to build leak-current suppression features, such as a rectification features, into the resistance-variable elements.